Adding S/P-DIF to your soundcard

This is something really simple I did a while ago…but wanted to show you nonetheless.

Most current notebooks still don’t provide an optical or even coaxial digital sound output, also known as S/P-DIF. This is something I can’t really relate to, as it might well be the only way to get encoded surround sound out of that thing without using an extra USB sound card or some crappy stereo upmixer. HDMI is also not the key to the problem since the digital audio stream is tightly embedded into the rest of the signal – an extractor circuit costs about 300 Euros over here!

Instead, save that time and take a look at your soundchip. Most feature S/P-DIF (especially if the containing device features HDMI) natively, but the corresponding pins are simply not connected to anything – which does not mean they are not active! Try to google for the part number (here ALC262), for most chips you will find datasheets right away. Once downloaded, head straight for the pin assignments section and keep an eye out for descriptors like “SPDIFO”.

Edit: If an output pin is available, chances are pretty good that it is useable even if the digital output is not shown in the mixer program. Some applications can turn off the optical output by software, but mostly that means the transmitter device while the data keeps on streaming.

In this case, the original docking station had the optical output built in even though the pin was left unconnected within the notebook, which made things a little easier – although it once again proves that manufacturers sometimes abandon pre-planned features for whatever reasons, which I as a customer generally don’t approve of.

Finding the corresponding pin on the dock connector was done using a generic multimeter. There should be no transistors or other obstructing parts in the signal path as most optical transmitters have built-in logic that only requires a TTL signal to switch the LED emitter. Out of the three or four pins on the transceiver device you should be able to identify one as ground, one as VCC and one of the remaining two will be the signal input. The remaining fourth one (if present) is most likely an enable pin and is tied to either VCC or GND.

Realtek ALC262 with upgrade

Realtek ALC262 with upgrade

Once the correct pin on the package is determined, carefully solder a thin enameled wire on top of it. This is the only tricky part and requires a thin tip and steady hands. Try not to heat it up for too long and check for accidental connections between the neighboring pins afterwards.

Aaaand you’re done. The signal you just tapped can be fed into an optical transmitter (e.g. TOTX… type) directly. By the way: If you are building/designing a digital to analog converter for audio purposes, this trick can be used to implement a very simple USB connection. Just get one of those cheap USB-plug soundcards. The chips inside mostly feature digital outputs and can be wired into an open TTL-compatible input of your DAC project.

Theme change

Still working on the designs, might switch themes some in the next days but I think I’ll stay with a bright one like this for contrast and readability.

You’ve got mail!

Two days ago, a small package arrived on my doorstep after 15 day’s worth of travel. Straight from China, 5 pc. of the L7251 spindle/VCM driver (remember the bad harddrive?), sold by Shenzhen Drivestar, who have a very friendly and competent customer service. Thanks again! I figured replacing the chip was the best option, looking at the chances of success.

L7251Well what do you know. After soldering in one of these, the little sucker spins again. It is not accessible, yet, but this is definately progress! Even the o’scope shows perfect waveforms at the motor terminals, but listening to the clicking and squeaking noises I’d say something is wrong with the heads.

After investigating a little further, I spotted the central problem: The drive has suffered a triple headcrash, meaning three of its trackheads have touched the disc surface prior to the fault and were literally ripped to shreds – but in the wrong direction! The head assembly was bent and got stuck near to the platter edges, *almost* in the standby rack. Something prevented it from getting there, as it should automatically move and lock there as soon as the disc loses power. My best guess is that the disc stack rotated against its normal rotation direction, which can happen if you carry the drive.

To explain: Normally, the heads would create their own air cushion using the movement of the platters and “float” over the surfaces during operation. While the platters are motionless, that cushion is not present and therefore there is friction between the surface and heads – and any motion of the platters is also directed at the heads. For that reason, the heads have to be parked before the disc slows down too far. Usually, some kind of mechanical tensioner handles that part. It also ensures that the head assembly never moves freely while the drive is stopped.

The resulting braking force during spinup probably caused the controller to fail, and after repairing that part, the disk spun and pushed the head out of the platter spindle.

To sum it up, it is very likely that the surface of the lower platters is severely damaged, and even if not – the only way to undo this is to mount a new head assembly. I will look into this a little further, but for the moment that’s just that until I get around to building myself a small “clean room box” for repairs on discs and displays.

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Minor setback

Somehow I knew that this went over too easy. Fortunately, magic smoke SMELLS. Just got the battery out in time, but part of the damage already happened.

Defective LED


Defective LED

...and another.

Some days ago, one of the triple-chains on the right half of the screen went out. While the darker area is clearly visible, the shadow is still illuminated by the neighboring LEDs. The cause is the LED in the second picture, the damage is only visible as a slight dark streak in the center of the yellow part. Then today the backlight fuse blew out of this world, and the reason is seen in the first picture. I knew this was a close call, but precisely “calibrating” the almost not existent isolation gap between LEDs and metal (by wedging a piece of paper in every here and there during glueing 😀 ) gave me no no reason for concern. Maybe some flexing or thermal deformation did the rest.

Anyhow, got to rebuild this part. Not too bad though, the stripes are inexpensive and I wanted to include a diffusor sheet anyway. As good a chance as any I guess.

Edit: By the way, the driver circuit survived the whole mess just fine. That IC is some tough design! Haven’t managed to fry a single one of those so far.

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